Branched vapor-grown carbon fiber, electrically conductive transparent composition and use thereof

ABSTRACT

A branched vapor-grown carbon fiber having an outer diameter of 0.5 μm or less and an aspect ratio of at least 10, the carbon fiber having a compressed specific resistance of 0.02 Ω·cm or less, each fiber filament having a hollow cylindrical structure, preferably the carbon fiber containing boron and having a compressed specific resistance of 0.018 Ω·cm or less. An electrically conductive transparent composition comprising a resin binder and carbon fiber incorporated into the binder, having transparency and comprising vapor grown carbon fiber having an outer diameter of 0.01-0.1 μm, an aspect ratio of 10-15,000, and a compressed specific resistance of 0.02 Ω·cm or less, and surface resistivity of 10,000 Ω/□ or less. An electrically conductive transparent material formed from the aforementioned electrically conductive transparent composition.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is an application filed pursuant toSection 111 (a) with a claim to priority to Provisional ApplicationSerial Nos. 60/267,176 and 60/267,179 filed Feb. 8, 2001 pursuant to 35U.S.C. Section 119(e) (1) in accordance with 35 U.S.C. 111(b).

TECHNICAL FIELD

[0002] The present invention relates to vapor grown carbon fiberexhibiting an enhanced function when used as an electrically conductiveor heat conductive filler for composite materials, such as resin- orrubber-based composite materials or an enhanced function when used as anadditive which may be incorporated into the electrodes of variousbatteries, such as lead storage batteries and to a process for producingit. The present invention also relates to an electrically conductivetransparent composition containing a resin and carbon fiber incorporatedinto the resin, which composition does not lose transparency inherent tothe resin and exhibits both electrical conductivity and transparency.The electrically conductive transparent composition of the presentinvention is useful as an electrically conductive transparent materialin a variety of materials requiring light transmission and electricalconductivity, for example, electrically conductive transparent coating,electrically conductive transparent film, or electrically conductivetransparent sheet.

BACKGROUND ART

[0003] In general, electrically conductive coating, film, or sheet isproduced from a mixture containing electrically conductive material andpaint or film material. Widely used electrically conductive materialsinclude metallic powder, electrically conductive inorganic oxide powder,and carbon powder. However, metallic powder has a drawback in that theelectrical conductivity of the powder is lowered through oxidation orcorrosion. Furthermore, when a noble metal (e.g., silver), which doesnot easily undergo oxidation or corrosion, is used for, for example,wires of an IC, etc., the noble metal involves problems, including shortcircuit due to migration. Although carbon powder does not have such adrawback of metallic powder, the electrical conductivity of carbonpowder is lower than that of metallic powder. Therefore, in order toenhance electrical conductivity, there have been proposed, for example,carbon fiber which is easily graphitized and has a specific structure inwhich an aspect ratio is large (Japanese Patent Publication (kokoku) No.06-39576), or a material containing entangled carbon fiber filaments(Japanese Patent Application Laid-Open (kokai) No. 07-102197).

[0004] However, in the case where the aforementioned electricallyconductive material is incorporated into a resin, a problem arises thattransparency inherent to the resin may be lost when the incorporationamount of the conductive material is increased in order to enhance theelectrical conductivity of the resin. For example, when a materialcontaining entangled carbon fiber filaments is incorporated into aresin, the incorporation amount of the material must be tens of mass %in order to secure sufficient enhancement of the electrical conductivityof the resin. As a result, when the thickness of a coating or a filmformed from the resin is about 1 mm, the transmittance of the coating orfilm becomes about 30%; i.e., the coating or film becomes opaque andbarely transmits light. In contrast, when the amount of carbon fiberincorporated into a resin is reduced in order to maintain thetransparency of the resin, the electrical conductivity of a coating orfilm formed from the resin is greatly reduced.

[0005] There has also been proposed an electrically conductivetransparent composition prepared from an electrically conductivematerial to which, in order to enhance electrical conductivity, amixture of graphite having an average particle size of 1-20 μm andcarbon powder having a BET specific surface area of 25-800 m²/g has beenincorporated (Japanese Patent Application Laid-Open (kokai) No.2000-173347). However, when the composition is formed to have athickness of 0.02-0.5 μm and a transmittance of 30%, the surfaceresistivity of the composition is 1×10⁵ Ω/□ (ohm/square)(or simplyreferred to Ω, hereinafter the same will do); i.e., the electricalconductivity of the composition is still low. As described above,conventional electrically conductive coating or electrically conductivefilm encounters difficulty in attaining both transparency and highelectrical conductivity.

[0006] An object of the present invention is to overcome theaforementioned problems of conventional electrically conductive coatingor electrically conductive film and to provide an electricallyconductive transparent composition comprising carbon fiber, inparticular vapor grown carbon fiber (hereinafter sometimes abbreviatedas “VGCF”), of very small outer diameter and high electricalconductivity, which composition does not lose transparency inherent to aresin and exhibits both transparency and high electrical conductivity;and an electrically conductive transparent material formed from thecomposition.

[0007] Vapor grown carbon fiber (VGCF) is produced by thermallydecomposing a raw material gas, such as hydrocarbon gas, in a vaporphase in the presence of a metallic catalyst, and by growing thedecomposition product into a fibrous shape. It has been known thatcarbon fiber having a diameter of tens of nm to 1,000 nm can be producedthrough this process.

[0008] A variety of processes for producing VGCF are disclosed,including a process in which an organic compound such as benzene,serving as a raw material, and an organic transition metal compound suchas ferrocene, serving as a catalyst, are introduced into ahigh-temperature reaction furnace together with a carrier gas, tothereby produce VGCF on a substrate (Japanese Patent ApplicationLaid-Open (kokai) No. 60-27700); a process in which VGCF is produced ina dispersed state (Japanese Patent Application Laid-Open (kokai) No.60-54998 (U.S. Pat. No. 4,572,813)); and a process in which VGCF isgrown on a reaction furnace wall by means of spraying onto the furnacewall droplets of a solution containing a raw material and a metalliccatalyst (Japanese Patent No. 2778434).

[0009] The aforementioned processes have enabled production of carbonfiber of relatively small outer diameter and high aspect ratio whichexhibits excellent electrical conductivity and heat conductivity and issuitable as a filler material. For example, carbon fiber having an outerdiameter of about 10 to about 200 nm and an aspect ratio of about 10 toabout 500 has been mass-produced and used, for example, as anelectrically conductive or heat conductive filler material to beincorporated into electrically conductive resin, or as an additive to beincorporated into lead storage batteries.

[0010] A characteristic feature of a VGCF filament resides in its shapeand crystal structure. A VGCF filament has a multi-layered shellstructure having a very thin central hollow portion, wherein a pluralityof carbon hexagonal network layers are grown around the hollow portionso as to form annual rings.

[0011] A carbon nano-tube, which is a type of carbon fiber having adiameter smaller than that of VGCF, has been discovered in soot obtainedby evaporating a carbon electrode through arc discharge in helium gas.The carbon nano-tube has a diameter of 1-30 nm, and has a structuresimilar to that of a VGCF filament; i.e., the tube has a hollowcylindrical structure having a central hollow portion, wherein aplurality of carbon hexagonal network layers are grown around the hollowportion so as to form annual rings. However, the process for producingthe nano-tube through arc discharge is not carried out in practice,since the process is not suitable for mass production.

[0012] Meanwhile, carbon fiber of high aspect ratio and highconductivity can be produced through the vapor-growth process, andtherefore various improvements to the carbon fiber have been made. Forexample, U.S. Pat. No. 4,663,230 and Japanese Patent Publication(kokoku) No. 3-64606 (European Patent No. 205556) disclose a graphiticcylindrical carbon fibril having an outer diameter of about 3.5 to about70 nm and an aspect ratio of at least 100. The carbon fibril has astructure such that a plurality of layers of ordered carbon atoms arecontinuously disposed concentrically around the longitudinal axis of thefibril, and the C-axis of each of the layers is substantiallyperpendicular to the longitudinal axis. The entirety of the fibril has asmooth surface, and includes no thermal carbon overcoat depositedthrough thermal decomposition. Japanese Patent Application Laid-Open(kokai) No. 61-70014 discloses vapor grown carbon fiber having an outerdiameter of 10-500 nm and an aspect ratio of 2-30,000, the thermaldecomposition carbon layer of the carbon fiber having a thickness of 20%or less the diameter of the carbon fiber. However, detailed studies havenot yet been performed on the branched hollow structure, compressedspecific resistance, and heat conductivity of the aforementioned carbonfibers.

[0013] Carbon fiber has low contact resistance, and, as compared withconventional carbon black or similar material, exhibits excellentelectrical conductivity and heat conductivity, and has high strength,since, in carbon fiber, carbon structure is developed along alongitudinal direction of a fiber filament, and fiber filaments areentangled extensively with one another. Therefore, various attempts havebeen made to enhance such characteristics of carbon fiber. For example,Japanese Patent No. 2862578 (European Patent No.491728) discloses thatthe contact resistance of carbon fiber is reduced by incorporating, intoa resin composition, carbon fiber containing entangled fiber filaments.Japanese Patent No. 1327970 discloses branched VGCF in which fresh VGCFis grown on a VGCF substrate. Japanese Patent Application Laid-Open(kokai) No. 6-316816 discloses VGCF having gnarled depositions thereon.

[0014] The aforementioned attempts have been made in order to ensurecontact between fine carbon fiber filaments in a composite material, bybringing the filaments into contact with one another or by bonding thefilaments with one another in advance. In addition to such carbon fiberfilaments, there has been a demand for a single carbon fiber filament ofenhanced electrical conductivity or heat conductivity.

DISCLOSURE OF THE INVENTION

[0015] The present inventors have improved the structure of VGCF, andhave obtained branched vapor-grown carbon fiber having a very smallouter diameter, each fiber filament having a hollow cylindricalstructure such that a central hollow portion extends throughout thefilament including a branched portion thereof; i.e., branchedvapor-grown carbon fiber of very small outer diameter exhibitingexcellent electrical conductivity and heat conductivity. The branchedvapor-grown carbon fiber has a very small outer diameter, each fiberfilament having a hollow cylindrical structure such that a centralhollow portion extends throughout the filament including a branchedportion thereof, the carbon fiber having high electrical conductivityand heat conductivity. When the carbon fiber is added to a material suchas resin or rubber or to electrodes of various batteries, the carbonfiber filaments are dispersed so as to form a network structure, tothereby enhance electrical conductivity and heat conductivity of such amaterial.

[0016] That is, the present invention provides a branched vapor-growncarbon fiber, a process for producing it, an electrically conductivetransparent composition and an electrically conductive transparentmaterial formed therefrom having the following constituent features.

[0017] 1. Branched vapor-grown carbon fiber having an outer diameter of0.5 μm or less and an aspect ratio of at least 10, each fiber filamenthaving a hollow cylindrical structure, characterized by having acompressed specific resistance of 0.02 Ω·cm or less;

[0018] 2. Branched vapor-grown carbon fiber according to 1 above, whichhas an outer diameter of 0.05-0.5 μm, a length of 1-100 μm, and anaspect ratio of 10-2,000;

[0019] 3. Branched vapor-grown carbon fiber according to 1 above, whichhas an outer diameter of 0.002-0.05 μm, a length of 0.5-50 μm, and anaspect ratio of 10-25,000;

[0020] 4. Branched vapor-grown carbon fiber according to 2 or 3 above,which has a compressed specific resistance of 0.018 Ω·cm or less, eachfiber filament having a structure such that a central hollow portionextends throughout the filament including a branched portion thereof;

[0021] 5. Branched vapor-grown carbon fiber according to 4 above, whichcomprises, in an amount of at least 10 mass %, branched carbon fiber,each fiber filament having a structure such that a central hollowportion extends throughout the filament including a branched portionthereof;

[0022] 6. Branched vapor-grown carbon fiber according to 1 above, whichfurther comprises boron;

[0023] 7. Branched vapor-grown carbon fiber according to 6 above, whichcomprises boron in an amount of 0.01-5 mass %;

[0024] 8. Branched vapor-grown carbon fiber according to any one of 1 to7 above, which has a heat conductivity of at least 100 kcal(mh° C.)⁻¹;

[0025] 9. Branched vapor-grown carbon fiber according to 8 above, whichhas a heat conductivity of at least 100 kcal(mh° C.)⁻¹ when the fiber iscompressed so as to attain a bulk density of 0.8 g/cm³;

[0026] 10. A process for producing branched vapor grown carbon fiberaccording to 1 above, by thermal decomposition of an organic compoundwith a transition metal catalyst, characterized by spraying droplets oforganic compound containing 5-10 mass % of a transition metal element orits compound on a heating furnace wall to allow reaction to form carbonfiber filaments on the furnace wall, burning the recovered filaments at800-1,500° C. in a non-oxidative atmosphere, and heating them at2,000-3,000° C. to perform graphitization treatment in a non-oxidativeatmosphere;

[0027] 11. A process according to 10 above, wherein the heating forgraphitization treatment is performed after doping with boron or atleast one boron compound selected from the group consisting of boronoxide, boron carbide, boric ester, boric acid or its salt, and organicboron compounds as a crystallization promotion compound in an amount of0.1-5 mass % in terms of boron;

[0028] 12. An electrically conductive transparent composition comprisinga resin binder and carbon fiber incorporated into the binder,characterized by having transparency and comprising vapor grown carbonfiber having an outer diameter of 0.01-0.1 μm, an aspect ratio of10-15,000, and a compressed specific resistance of 0.02 Ω·cm or less;

[0029] 13. An electrically conductive transparent composition accordingto 12 above, wherein the carbon fiber is vapor grown carbon fiber havingan outer diameter of 0.05-0.1 μm or less, a length of 1-100 μm, and anaspect ratio of 10-2,000, each fiber filament having a hollowcylindrical structure;

[0030] 14. An electrically conductive transparent composition accordingto 12 above, wherein the blending amount of vapor grown carbon fiber is5-40 mass % of the total composition;

[0031] 15. An electrically conductive transparent composition accordingto 12 above, which has a surface resistivity of 10,000 Ω/□ or less;

[0032] 16. An electrically conductive transparent composition accordingto 12 above, which has a surface resistivity of 5-10,000 Ω/□, and atransmittance of at least 60% when the composition is formed to have athickness of 0.5 μm;

[0033] 17. An electrically conductive transparent composition accordingto 12 or 13 above, wherein the carbon fiber is vapor grown carbon fiberhaving an interlayer distance (d₀₀₂) of carbon crystal layers of 0.339nm or less and a compressed specific resistance of 0.018 Ω·cm or less;

[0034] 18. An electrically conductive transparent composition accordingto 13 above, wherein the branched vapor grown carbon fiber has acompressed specific resistance of 0.018 Ω·cm or less, each fiberfilament thereof having a structure such that a central hollow portionextends throughout the filament including a branched portion thereof;

[0035] 19. An electrically conductive transparent composition accordingto 18 above, wherein the carbon fiber comprises, in an amount of atleast 10 mass %, branched vapor-grown carbon fiber, each fiber filamenthaving a structure in which a central hollow portion extends throughoutthe filament including a branched portion thereof;

[0036] 20. An electrically conductive transparent composition accordingto 12 or 13 above, wherein the vapor grown carbon fiber comprises boronor a combination of boron and nitrogen in an amount of 0.01-3 mass %;

[0037] 21. An electrically conductive transparent composition accordingto 12 or 13 above, wherein the vapor grown carbon fiber comprisesfluorine in an amount of 0.001-0.05 mass %;

[0038] 22. An electrically conductive transparent composition accordingto 12 or 13 above, wherein the vapor grown carbon fiber is coated with20-70 mass % aluminum oxide;

[0039] 23. An electrically conductive transparent composition accordingto 12 or 13 above, which comprises carbon black together with the vaporgrown carbon fiber;

[0040] 24. An electrically conductive transparent material formed froman electrically conductive transparent composition as recited in any oneof 12 through 23 above; and

[0041] 25. An electrically conductive transparent material according to24 above, which assumes a form of coating, film produced throughspraying, film, or sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1 shows a photomicrograph of the branched vapor-grown carbonfiber of the present invention as obtained by use of a transmissionelectron microscope (magnification: ×100,000).

[0043]FIG. 2 shows a photomicrograph of a branched portion of thebranched vapor-grown carbon fiber of the present invention(magnification: ×100,000).

[0044]FIG. 3 shows a photomicrograph of the conventional branchedvapor-grown carbon fiber as obtained by use of a transmission electronmicroscope (magnification: ×100,000).

[0045]FIG. 4 is a schematic longitudinal cross-section showing a cellfor the measurement of compressed specific resistance of the carbonfiber of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0046] First of all, the branched vapor-grown carbon fiber of thepresent invention will be explained.

[0047] The present invention provides a branched carbon fiber producedthrough the vapor-growth process, which has an outer diameter of 0.5 μmor less, an aspect ratio of at least 10, and a compressed specificresistance of 0.02 Ω·cm or less, each fiber filament having a hollowcylindrical structure. Preferably, the branched vapor-grown carbon fiberhas a compressed specific resistance of 0.018 Ω·cm or less, each fiberfilament having a structure such that a central hollow portion extendsthroughout the filament including a branched portion thereof.

[0048] As shown in photomicrographs of FIGS. 1 and 2 (magnification:×100,000), in preferred branched vapor-grown carbon fiber of the presentinvention, each fiber filament has a structure such that a centralhollow portion extends throughout the filament including a branchedportion thereof. As a result, the sheath-forming carbon layers of thecarbon fiber assume uninterrupted layers so that although the filamentsof the carbon fiber have a very small diameter, the carbon fiberexhibits excellent electrical conductivity and heat conductivity. Theelectrical conductivity and heat conductivity of a conventional carbonfiber vary with the degree of contact or adhesion between the fiberfilaments. Since branched portions of the conventional carbon fiber arebonded with one another so as to form nodules as shown in, for example,the photomicrograph of FIG. 3 (magnification: ×100,000), the electricalconductivity and heat conductivity of the conventional carbon fiber arelower than those of the carbon fiber of the present invention.

[0049] As used herein, the term “hollow cylindrical structure” of thebranched vapor-grown carbon fiber refers to a structure such that aplurality of carbon layers form a sheath. The hollow cylindricalstructure encompasses a structure such that sheath-forming carbon layersform an incomplete carbon sheet; a structure such that the carbon layersare partially broken; and a structure such that the laminated two carbonlayers are formed into a single carbon layer. The cross section of thesheath does not necessarily assume a round shape, and may assume anelliptical shape or a polygonal shape. No particular limitation isimposed on the interlayer distance (d₀₀₂) of carbon crystal layers. Theinterlayer distance (d₀₀₂) of the carbon layers as measured throughX-ray diffraction is preferably 0.339 nm or less, more preferably 0.338nm or less. The thickness (Lc) of the carbon crystal layer in the c axisdirection is preferably 40 nm or less.

[0050] The branched vapor-grown carbon fiber of the present inventionhas a very small diameter; i.e., an outer diameter of 0.5 μm or less,and an aspect ratio of at least 10. Preferably, the carbon fiber has anouter diameter of 0.05-0.5 μm and a length of 1-100 μm (i.e., an aspectratio of 10-2,000); or an outer diameter of 0.002-0.05 μm and a lengthof 0.5-50 μm (i.e., an aspect ratio of 10-25,000). When the outerdiameter of the carbon fiber exceeds 0.5 μm, mixing of the carbon fiberin the resin is difficult, which is not preferable. In contrast, whenthe outer diameter of the carbon fiber is less than 0.002 μm, thestrength of the carbon fiber is lowered, allowing the fiber to breakeasily, which is not preferable.

[0051] Although carbon fiber having an outer diameter of 0.05-0.5 μm anda length of 1-100 μm can be produced through the process for producingbranched vapor-grown carbon fiber (Japanese Patent No. 2778434) in whichdroplets of a solution including a raw material and a metallic catalystare sprayed onto a reaction furnace wall, the carbon fiber of thepresent invention has an outer diameter smaller than that of the abovecarbon fiber by one digit; i.e., an outer diameter of 0.01-0.1 μm. Sucha very thin carbon fiber can be produced by utilizing the catalyticaction of a crystallization promotion element, preferably boron, etc.;i.e., by doping (adding a small amount of) this element to carboncrystals, during graphitization of deposited carbon fiber. The dopingamount of the element in terms of boron is suitably 0.01 to 5 mass %,preferably 0.1 to 3 mass %. When the amount of boron exceeds 5 mass %,doping with boron is difficult, whereas when the amount of boron is lessthan 0.01 mass %, the effect of boron is not satisfactory. When boron isincorporated into carbon crystals, the interlayer distance (d₀₀₂) ofcarbon layers is reduced, allowing crystallization to proceed.

[0052] The branched vapor-grown carbon fiber of the present inventionhas a compressed specific resistance when the fiber is compressed so asto attain a bulk density of 0.8 g/cm³ (hereinafter the resistance willbe simply referred to as “compressed specific resistance”) of 0.02 Ω·cmor less, preferably 0.018 Ω·cm or less. As described below in Examples,carbon fiber including branched fiber, which is produced through theconventional vapor-growth process, has a compressed specific resistanceof about 0.021 Ω·cm. When such a conventional carbon fiber is mixed witha resin to thereby prepare a conductive paste, the volume resistance ofthe paste is on the order of 0.38-0.45 Ω·cm. In contrast, as shown inExamples, the carbon fiber including branched fiber of the presentinvention has an electrical conductivity higher than that of theconventional carbon fiber, and has a compressed specific resistance of0.005-0.018 Ω·cm.

[0053] The branched vapor-grown carbon fiber of the present inventionhas a heat conductivity of at least 100 kcal(mh° C.)⁻¹, or a heatconductivity when the fiber is compressed so as to attain a bulk densityof 0.8 g/cm³ of at least 100 kcal(mh° C.)⁻¹. Since the carbon fiber hasa branched shape and enhanced crystallinity, when the fiber is mixedwith a resin, the heat conductivity of the resultant composite materialcan be enhanced. In order to obtain the effect of the branchedvapor-grown carbon fiber, the carbon fiber is preferably incorporatedinto a resin in an amount of at least 10 mass %. Heat conductivitycorrelates to electrical conductivity; i.e., when electricalconductivity is high, heat conductivity is also high.

[0054] The aforementioned branched vapor-grown carbon fiber of thepresent invention can be used, in a variety of fields, as a material forresin filler for use in magnetic wave shielding materials and antistaticmaterials, conductive ink, a conductive paste, a transparent electrode,electrode additive, conductivity imparting agent for photoconductordrums, optical material, high-strength-structure material, and heatconductive material.

[0055] The process for producing the branched vapor-grown carbon fiberof the present invention will next be described.

[0056] Branched vapor grown carbon fiber of the present invention can beproduced according to the process for producing vapor-grown carbon fiber(Japanese Patent No. 2778434) in which droplets of a solution includinga raw material and a metallic catalyst are sprayed onto a reactionfurnace wall.

[0057] First, crude fine carbon fiber filaments are obtained by thermaldecomposition of an organic compound, in particular a hydrocarbon, byuse of an organic transition metal compound serving as a catalyst.

[0058] The organic transition metal compound as used herein includesorganic compounds that contain metals belonging to the Group IVa, Va,VIa, VIIa and VIII in the periodic table. Among them, those compoundssuch as ferrocene and nickelocene are preferred.

[0059] In order to increase the content of branched carbon fiber, theconcentration of a metallic catalyst such as ferrocene, which is addedto a raw material, is preferably increased. Conventionally, theconcentration of the metallic catalyst is about 4 mass %, but in thepresent invention the concentration of the metallic catalyst ispreferably 5-10 mass %, more preferably about 7 mass %.

[0060] In addition, a sulfur compound may be used as a promoter. Theform of the sulfur compound is not particularly limited as far as it isdissolved in an organic compound as a carbon source. The sulfur compoundthat can be used includes thiophene, various types of thiols, inorganicsulfur and so forth. The use amount thereof is suitably 0.01-10.0 mass%, preferably 0.03-5.0 mass %, more preferably 0.1-4.0 mass %.

[0061] The organic compound that can be used as a raw material for thecarbon fiber includes organic compounds such as benzene, toluene,xylene, methanol, ethanol, naphthalene, phenanthrene, cyclopropane,cyclopentane and cyclohexane; volatile oils; kerosene; or gases such asCO, natural gas, methane, ethane, ethylene and acetylene, and mixturesthereof. Among then, aromatic compounds such as benzene, toluene andxylene are particularly preferred.

[0062] Usually, hydrogen gas and other reducing gases are used as acarrier gas. It is preferred that the carrier gas be preliminarilyheated at 500-1,300° C. The reason for heating is that both generationof a catalyst metal and supply of a carbon source through thermaldecomposition of the carbon compound can take place simultaneously sothat the reaction can complete instantaneously to obtain a finer carbonfiber. When the carrier gas is mixed with the raw material, the thermaldecomposition of the carbon compound as a raw material can barely occurif the temperature for heating the carrier gas is below 500° C. while ifsuch heating temperature exceeds 1,300° C., the carbon fiber grows inthe radial direction, so that the diameter tends to become larger.

[0063] The use amount of carrier gas is suitably 1-70 mol per mol of thecarbon source (organic compound). The diameter of the carbon fiber canbe controlled by varying the ratio of the carbon source to the carriergas.

[0064] The raw material is prepared by dissolving a transition metalcompound and a sulfur compound as a promoter in an organic compound as acarbon source.

[0065] There has been conventionally known a process for producingbranched carbon fiber in which a raw material and a metallic catalystare gasified, and fed to a reaction furnace. However, this conventionalprocess can barely generate branched carbon fiber. Accordingly, in thepresent invention it is preferred that a solution including an organiccompound raw material such as benzene and a metallic catalyst such asferrocene be sprayed and fed in the form of a liquid into the reactionfurnace or a portion of the carrier gas be used as a purge gas to gasifythe solution before it can be fed into the reaction furnace. In order toobtain carbon fiber having a smaller diameter, it is preferably that agas obtained by gasifying the solution is fed into the reaction furnace.When the solution is sprayed in the form of a liquid onto a reactionfurnace wall to thereby allow reaction to proceed, the concentration ofthe raw material and the metallic catalyst increases locally, and thusbranched carbon fiber is easily deposited. Through recovery andcrystallization of the thus-deposited carbon fiber, there can beproduced branched vapor-grown carbon fiber containing, in an amount ofat least 10 mass %, branched carbon fiber filaments having a structurein which central hollow portions extend throughout the filamentsincluding branched portions thereof.

[0066] As the reaction furnace, usually a vertical type electric furnaceis used. The temperature of reaction furnace is 800-1,300° C.,preferably 1,000-1,300° C. By feeding the raw material solution and thecarrier gas, or the raw material gas obtained by gasifying the rawmaterial and the carrier gas to the reaction furnace the temperature ofwhich has been elevated to a predetermined temperature to allow them toreact with each other to obtain carbon fiber.

[0067] After the carbon fiber containing branched carbon fiber filamentsproduced in the reaction furnace is recovered, the carbon fiber isheated and fired at 800-1,500° C. in a non-oxidizing atmosphere such asargon gas, to thereby allow crystallization to proceed. Subsequently,the thus-crystallized carbon fiber is further heated at 2,000-3,000° C.in a non-oxidizing atmosphere, to thereby allow graphitization toproceed. During this graphitization, the crystallized carbon fiber isdoped with a crystallization promotion element (by addition of a smallamount of it), to thereby enhance crystallinity of the fiber. Thecrystallization promotion element is preferably boron. Since thegraphitized fine carbon fiber is covered with a dense basal plane (aplane of hexagonal network structure), preferably, carbon fiber of lowcrystallinity, which has been heated at 1,500° C. or lower is doped withboron. In this case, also carbon fiber of high crystallinity can beobtained since the carbon fiber of low crystallinity is heated to itsgraphitization temperature when it is doped with boron; i.e., when it issubjected to boronization.

[0068] The doping amount of boron is typically 5 mass % or less withrespect to the amount of carbon. When carbon fiber is doped with boronin an amount of 0.1-5 mass % in terms of boron, the crystallinity of thecarbon fiber can be effectively enhanced. Therefore, elementary boron ora boron compound (e.g., boron oxide (B₂O₃), boron carbide (B₄C), a boricester, boric acid (H₃BO₃) or a salt thereof, or an organic boroncompound) as a crystallization promotion compound is added to carbonfiber such that the boron content of the carbon fiber falls within theabove range. In consideration of percent conversion, the boron compoundmay be added in an amount of 0.1-5 mass % in terms of boron with respectto the amount of carbon. It should be noted, however, that onlyrequirement is that boron be present when the fiber is crystallizedthrough heat treatment. Boron may be evaporated during the course ofhigh-temperature treatment performed after carbon fiber has been highlycrystallized, to thereby reduce the boron content of the carbon fiberrelative to the amount of boron initially added to the fiber. However,such a reduction is acceptable only to such an extent that the amount ofresidual boron in the fiber after the treatment is about 0.01 mass % ormore.

[0069] The temperature required for introducing boron into carboncrystals or the surface of carbon fiber is at least 2,000° C.,preferably at least 2,300° C. When the heating temperature is lower than2,000° C., introduction of boron becomes difficult, because of lowreactivity between boron and carbon. In order to enhance crystallinityof carbon fiber, and to make the interlayer distance (d₀₀₂) of carboncrystal layers 0.338 nm or less, the heating temperature is preferablymaintained at 2,300° C. or higher. The heat treatment is carried out ina non-oxidizing atmosphere, preferably in an atmosphere of rare gas suchas argon. When the heat treatment is carried out for a very long periodof time, sintering of carbon fiber proceeds, resulting in a low yield.Therefore, after the temperature of the center portion of carbon fiberreaches the target temperature, the carbon fiber is maintained at thetarget temperature within about one hour.

[0070] Carbon fiber produced through the vapor-growth process has a verysmall bulk density. Therefore, preferably, after the carbon fiber isuniformly mixed with boron or a boron compound, the resultant mixture issubjected to shaping, granulation, or compression, and the resultantcarbon fiber of high density is heated. When carbon fiber of highdensity is subjected to heat treatment, a portion of the fiber issintered to become flocky. Therefore, after the flocky portion ispulverized, the carbon fiber is used in a variety of materials.

[0071] Next, electrically conductive transparent composition of thepresent invention will be explained.

[0072] The electrically conductive transparent composition of thepresent invention contains a binder formed from a resin, particularly atransparent resin, and carbon fiber incorporated into the binder. Acharacteristic feature of the composition resides in that thecomposition contains vapor grown carbon fiber having an outer diameterof 0.01-0.1 μm, an aspect ratio of 10-15,000, and a compressed specificresistance of 0.02 Ω·cm or less, and that the composition has a surfaceresistivity of 10,000 Ω/□ or less. The composition of the presentinvention has both transparency and high electrical conductivity and isused as a transparent electrode for coating, film produced throughspraying, film, or sheet.

[0073] The carbon fiber used in the electrically conductive transparentcomposition of the present invention is produced through thevapor-growth process. As aforementioned, vapor grown carbon fiber (VGCF)is produced by thermally decomposing a raw material gas, such ashydrocarbon gas, in a vapor phase in the presence of a metalliccatalyst, and by growing the decomposition product into a fibrous shape.A variety of processes for producing VGCF are disclosed, including aprocess in which an organic compound such as benzene, serving as a rawmaterial, and an organic transition metal compound such as ferrocene,serving as a catalyst, are introduced into a high-temperature reactionfurnace together with a carrier gas, to thereby produce VGCF on asubstrate (Japanese Patent Application Laid-Open (kokai) No. 60-27700);a process in which VGCF is produced in a dispersed state (JapanesePatent Application Laid-Open (kokai) No. 60-54998); and a process inwhich VGCF is grown on a reaction furnace wall by means of spraying ontothe furnace wall droplets of a solution containing a raw material and ametallic catalyst (Japanese Patent No. 2778434). The aforementionedprocesses have enabled production of, for example, VGCF having an outerdiameter of about 0.01 to about 0.5 μm and an aspect ratio of about 10to about 500.

[0074] In the present invention, the carbon fiber used is vapor growncarbon fiber having an outer diameter of 0.01-0.1 μm and an aspect ratioof 10-15,000. When carbon fiber having an outer diameter of more than0.1 μm is incorporated into a resin, the transparency of the resin isgreatly lowered. In contrast, when the outer diameter of carbon fiber isless than 0.01 μm, the strength of the carbon fiber is reduced, and thuswhen the carbon fiber is incorporated into a resin, the fiber is easilybroken. Meanwhile, when the aspect ratio of carbon fiber is more than15,000; i.e., when carbon fiber is very long, fiber filaments areexcessively entangled and as a result uniform dispersion of the carbonfiber in a resin becomes difficult.

[0075] Carbon fiber having an outer diameter of 0.05-0.5 μm and a lengthof 1-100 μm can be produced through the process for producingvapor-grown carbon fiber (Japanese Patent No. 2778434) in which dropletsof a solution including a raw material and a metallic catalyst aresprayed onto a reaction furnace wall. However, the electricallyconductive transparent composition of the present invention employscarbon fiber having an outer diameter of 0.01-0.1 μm. In order to obtaina very fine carbon fiber having further improved crystallinity, thedeposited carbon fiber may be graphitized. In this case, utilizing thecatalytic action of a crystallization promotion element, e.g., boron ora combination of boron and nitrogen; i.e., by doping carbon crystals orthe surface of carbon fiber with such an element, graphitized carbonfiber can be obtained. The doping amount of such an element is 0.01-5mass %, preferably 0.1-3 mass %, more preferably 0.2-2.0 mass %. Whenthe amount of such an element exceeds 5 mass %, doping with the elementis difficult, whereas when the amount of the element is less than 0.01mass %, the effect of the element is not satisfactory. When such anelement as boron is incorporated into carbon crystals, the interlayerdistance (d₀₀₂) of carbon layers is reduced, allowing crystallization toproceed. As a result, there can be produced carbon fiber having, ascompared with conventional carbon fiber, a small outer diameter, highelectrical conductivity, and high dispersibility to a resin.

[0076] The vapor grown carbon fiber used in the electrically conductivetransparent composition of the present invention has a compressedspecific resistance of 0.02 Ω·cm or less, preferably 0.018 Ω·cm or less,more preferably 0.015 Ω·cm or less. Incidentally, the carbon fiberproduced through the conventional vapor-growth process has a compressedspecific resistance of about 0.021 Ω·cm. In contrast, the carbon fiberused in the present invention has an electrical conductivity higher thanthat of the conventional carbon fiber, and has a compressed specificresistance of, for example, 0.005-0.018 Ω·cm. When carbon fiber having acompressed specific resistance of more than 0.02 Ω·cm is used, obtaininga transparent composition having a surface resistivity of 10,000 Ω/□, orless is difficult.

[0077] The vapor grown carbon fiber preferably used in the electricallyconductive transparent composition of the present invention is branchedvapor grown branched carbon fiber as described above that contains alarge amount of branched carbon fiber, each fiber filament having astructure such that a central hollow portion extends throughout thefilament including a branched portion thereof. In such a vapor-grown,branched carbon fiber filament having a hollow cylindrical structure,sheath-forming carbon layers assume uninterrupted layers. Therefore,although having a very small diameter, the branched carbon fiberexhibits excellent electrical conductivity and heat conductivity. Theelectrical conductivity and heat conductivity of conventional carbonfiber vary with the degree of contact or adhesion between fiberfilaments. Since branched portions of the conventional carbon fiber arebonded with one another so as to form nodules, the electricalconductivity and heat conductivity of the conventional carbon fiber arelow as compared with the present branched carbon fiber, each fiberfilament having a structure such that a central hollow portion extendsthroughout the filament including a branched portion thereof.

[0078] The vapor grown carbon fiber used in the electrically conductivetransparent composition of the present invention may be treated withfluorine so as to contain 0.001-0.05 mass % fluorine. The fluorinetreatment is performed, for example, by performing contact treatment at0-200° C. in the presence of a fluorine containing gas (F₂, HF, etc) orby plasma treatment with a fluorinated lower hydrocarbon such as CF₄(for example, Japanese Patent Application Laid-open (Kokai) No.8-31404). When the carbon fiber is treated with fluorine, the repellencyof the surface of the carbon fiber is enhanced. As a result, carbonfiber filaments are not easily flocculated, and dispersibility of thecarbon fiber can be enhanced. When the fluorine content is less than0.001 mass %, the effect of fluorine treatment is unsatisfactory,whereas when the fluorine content exceeds 0.05 mass %, carbon crystalplanes are broken, and the surface of the carbon fiber becomes rough.

[0079] The vapor grown carbon fiber used in the present invention may betreated with an aluminum compound (e.g., alumina gel, aluminum chloride,aluminum sulfate, aluminum nitrate, aluminum silicate, an aluminate, analuminic ester, or aluminum hydroxide), preferably with alumina gel,aluminum silicate, an aluminate, or aluminum hydroxide, and may becoated with 20-70 mass % aluminum oxide. For example, an alumina filmmay be formed on the surface of fiber by activating the vapor growncarbon fiber with carbon dioxide to form activated carbon fiber, dippingit in about 10% sulfuric acid and washing with water, adding the fiberin an aluminum compound solution.

[0080] When the carbon fiber is subjected to such surface treatment, thehydrophilicity of the surface of the carbon fiber can be enhanced. As aresult, adhesion between the carbon fiber and a resin is enhanced, andthe dispersibility of the carbon fiber is enhanced. When the coatingamount is less than 20 mass %, the effect of coating is unsatisfactory,whereas when the coating amount exceeds 70 mass %, adhesion betweencarbon fiber filaments is increased.

[0081] In the electrically conductive transparent composition of presentinvention, a suitable incorporation amount of vapor grown carbon fiberis appropriately 5-40 mass %, preferably 5-20 mass %, on the basis ofthe entirety of the composition. When the incorporation amount fallswithin the above range, the composition has high transparency and highelectrical conductivity. Specifically, the composition has a surfaceresistivity of 10,000 Ω/□ or less, and can attain a transmittance of 70%or more when the composition is formed to have a thickness of 0.5 μm. Inthis connection, conventional electrically conductive coatings to whichcarbon black as a sole carbonaceous material is incorporated in anamount nearly equal to that of the corresponding carbonaceous materialincorporated into the present composition have a transmittance of 30% orless; i.e., light-penetrability of the coatings is very low; whereas,when the amount of carbon black incorporated into such conventionalcoatings is reduced such that the light-penetrability of the coatings ismaintained at a level comparable to that of a coating formed from thepresent composition, the surface resistivity of the conventionalcoatings becomes 20,000 Ω/□ or more; i.e., the electrical conductivityof the coatings is greatly reduced.

BEST MODE FOR CARRYING OUT THE INVENTION

[0082] Hereinafter, the present invention will be illustrated by way ofexamples and comparative examples. However, the present invention shouldnot be construed as being limited by the following description.

[0083] In the following examples, branched fiber content (area %), boroncontent, bulk density (tapping density) (g/cm³), compressed specificresistance (Ω·cm), specific resistance of paste (Ω·cm), surfaceresistivity of coating (Ω/□), transmittance (%) were measured by thefollowing processes.

[0084] 1) Branched Fiber Content (Mass %):

[0085] In a photograph showing cross section of carbon fiber by use oftransmission electron microscope (TEM), a ratio of the cross sectionalarea of branched carbon fiber filaments to the total cross sectionalarea of carbon fiber filaments was obtained and assuming that thespecific density is 1, the ratio was defined as mass %.

[0086] 2) Boron Content:

[0087] Powder sample of carbon fiber to which calcium carbonate wasadded was incinerated at 800° C. in an oxygen flow. Then, after addingcalcium carbonate, the obtained ash was heat-molten and the melt wasdissolved in water. The resultant aqueous solution was subjected toquantitative analysis by use of inductively coupled plasma (ICP)emission spectral analysis.

[0088] 3) Bulk Density (Tapping Density) (g/cm³):

[0089] A predetermined amount (6.0 g) of sample was weighed and placedin a 15-mmφ cell for measurement, which was set in a tapping apparatus.At a falling height of 45 mm and a tapping speed of 2 second/time, thesample was freely fallen 400 times. Thereafter, the volume of the samplewas measured. From the relationship between the volume and mass, thebulk density of the sample was calculated.

[0090] 4) Compressed Specific Resistance (Ω·cm):

[0091] Sample to be measured was placed in a resin cell 4 as shown inFIG. 4 and pressed by compression rods 2 from above and below andcurrent was applied at a constant pressure. Then the voltage betweenterminals for the measurement of voltage placed at a midpoint of thesample was read and specific resistance was calculated from the crosssectional area of the vessel and the distance between the voltageterminals. The specific resistance varied depending on the pressingconditions and it showed a higher resistance at a lower pressure whereasas the pressure is further increased above a certain pressure, it showeda substantially constant value regardless of the pressing conditions. Inthe present invention, the value obtained when the sample was compactedto a bulk density of 0.8 g/cm³ by the following operation was defined asvolume specific resistance (compressed specific resistance).

[0092] That is, a predetermined amount of sample was placed in a cell 4for the measurement of compressed specific resistance made of a resinhaving a planar area of 1×4 cm² and a depth of 10 cm, provided with acopper plate current terminal 3 for applying current to an object 5 tobe measured and with a voltage measuring terminals 1 at a midpoint, andthe sample was increasingly compressed by the compression rod 2 fromabove and while measuring the compression a current of 0.1 A was appliedthereto. When the bulk density of 0.8 g/cm³ was reached, the voltage (E)V between the two terminals 1 for the measurement of voltage at adistance of 2.0 cm therebetween inserted through the bottom of thevessel was read. Specific resistance (R) (Ω·cm) was calculated accordingto the following formula.

R(Ω·cm)=(E/0.1)×D(cm ²)/2(cm)

[0093] In the above formula, D represents a cross sectional area(depth×width) of powder in the direction of current=10d.

[0094] 5) Surface Resistivity of Coating Film (Ω/□):

[0095] A coating film was prepared and measured according to 4-terminalmethod in compliance with “JIS K7194” by use of Lotest Hp MCP-T410manufactured by Mitsubishi Chemical, Inc.

[0096] 6) Specific Resistance of Paste (Ω·cm):

[0097] A paste film sample having a film thickness of 25 μm was preparedby use of a doctor blade and the surface resistivity of the sample wasmeasured according to 5) above. The obtained value was divided by thefilm thickness to obtain specific resistance of the paste.

[0098] 7) Transmittance (%):

[0099] This was measured by integrating-sphere light transmittancemethod in compliance with “JIS K7105” by use of NDH-1001 DP manufacturedby Nippon Denshoku Industries Co., Ltd.

EXAMPLE 1

[0100] In accordance with the description in Japanese Patent No.2778434, vapor grown carbon fiber was produced through a productionprocess in which ferrocene (7 mass %) was dissolved in benzene, anddroplets of the resultant solution were sprayed onto a furnace wall, tothereby cause thermal decomposition of the solution. The thus-producedcarbon fiber was heated at 1,200° C. in an argon atmosphere, and furtherheated at 2,800° C. in an argon atmosphere. After heat treatment wascompleted, the resultant flocky carbon fiber was pulverized, to therebyyield vapor grown carbon fiber having an outer diameter of 0.1-0.2 μm, alength of 10-20 μm, and an aspect ratio of 50-200. Through observationby use of a transmission electron microscope (TEM), the carbon fiber wasfound to contain branched carbon fiber in an amount of 22 mass %. Thebulk density (tapping density) of the carbon fiber was 0.035 g/cm³.After the carbon fiber was compressed so as to attain a bulk density of0.8 g/cm³, the resultant carbon fiber had a compressed specificresistance (powder resistance) of 0.018 Ω·cm. The carbon fiber (40 mass%) was mixed with polyurethane, to thereby prepare a paste. The specificresistance of the paste was 0.25 Ω·cm. The results are shown in Table 1.FIG. 1 shows a photomicrograph (magnification: ×100,000) of a branchedportion of the carbon fiber.

EXAMPLE 2

[0101] Boron carbide (B₄C) powder (4 mass %) was added to vapor growncarbon fiber containing branched carbon fiber produced in a mannersimilar to that of Example 1, and uniform mixing was carried out. Theresultant mixture was placed in a graphitic crucible, compressed, andthen heated at 2,700° C. in an argon atmosphere for 60 minutes. Theresultant product was pulverized, to thereby yield boron-containingvapor grown carbon fiber containing branched carbon fiber. The boroncontent of the carbon fiber was 1.8 mass %. In a manner similar to thatof Example 1, the bulk density and compressed powder specific resistanceof the carbon fiber, and the specific resistance of a resin pastecontaining the fiber were measured. The bulk density (tapping density)was 0.036 g/cm³; the compressed powder specific resistance was 0.005Ω·cm; and the specific resistance of the resin paste was 0.08 Ω·cm. Theresults are shown in Table 1. Further, FIG. 2 shows a photomicrograph(magnification: ×100,000) of a branched portion of the carbon fiber. Thestate was observed in which a central hollow portion extends throughoutthe filament including a branched portion thereof.

EXAMPLES 3 AND 4

[0102] Boron carbide (B₄C) powder was added to vapor grown carbon fibercontaining branched carbon fiber produced in a manner similar to that ofExample 1, and uniform mixing was carried out. The resultant mixture wasplaced in a graphitic crucible, compressed, and then heated at2,800-2,900° C. in an argon atmosphere for 60 minutes, to thereby yieldboron-containing vapor grown carbon fiber containing branched carbonfiber. In Example 3, the boron content of the carbon fiber was 0.5 mass%, and in Example 4, the boron content of the carbon fiber was 0.2 mass%. In a manner similar to that of Example 1, the bulk density andcompressed powder specific resistance of the carbon fiber, and thespecific resistance of a resin paste containing the fiber were measured.The results are shown in Table 1 together with the boron content.

COMPARATIVE EXAMPLE 1

[0103] Vapor grown carbon fiber (outer diameter: 0.1-0.2 μm, length:10-20 μm) was produced through, instead of the production processdescribed in Example 1, a conventional production process in which a rawmaterial was gasified and then fed into a furnace. Through observationby use of a TEM, the thus-produced carbon fiber was found to containonly a small amount of branched carbon fiber. In a manner similar tothat of Example 1, the bulk density and compressed powder specificresistance of the carbon fiber, and the specific resistance of a resinpaste containing the fiber were measured. The results are shown in Table1.

COMPARATIVE EXAMPLE 2

[0104] The procedure of Example 1 was repeated, except that the amountof ferrocene added was reduced to 2 mass %, to thereby produce vaporgrown carbon fiber (outer diameter: 0.1-0.2 μm, length: 5-10 μm).Through observation by use of a TEM, the carbon fiber was found tocontain branched carbon fiber in an amount of 5 mass %. In a mannersimilar to that of Example 1, the bulk density and compressed powderspecific resistance of the carbon fiber, and the specific resistance ofa resin paste containing the fiber were measured. The results are shownin Table 1. FIG. 3 shows a photomicrograph of a branched portion of thecarbon fiber. TABLE 1 Branched Compressed Specific Carbon fiber (μm)carbon Bulk specific resistance Outer fiber Boron density resistance ofpaste diameter Length content content (g/cm³) (Ω · cm) (Ω · cm) Example1 0.1-0.2 10-20 22 0 0.035 0.018 0.25 2 0.1-0.2 10-20 22 1.8 0.036 0.0050.08 3 0.1-0.2 10-20 22 0.5 0.035 0.005 0.09 4 0.1-0.2 10-20 22 0.20.036 0.005 0.08 Comparative 1 0.1-0.2 10-20 0 0 0.035 0.022 0.45Example 2 0.1-0.2  5-10 5 0 0.035 0.021 0.38

[0105] As shown in Table 1, the branched vapor-grown carbon fiber of thepresent invention (Examples 1 through 4) has a compressed powderspecific resistance of 0.02 Ω·cm or less, which is lower than that ofthe conventional vapor grown carbon fiber (Comparative Examples 1 and2). Therefore, the specific resistance of a resin paste containing thepresent branched vapor-grown carbon fiber is low; i.e., 0.3 Ω·cm orless. In contrast, the conventional vapor grown carbon fiber(Comparative Examples 1 and 2) has a compressed powder specificresistance of higher than 0.02 Ω·cm. Since the present branchedvapor-grown carbon fiber containing boron has high crystallinity, itscompressed powder specific resistance is further reduced.

[0106] Furthermore, as shown in the photomicrographs (magnification:×100,000) in FIGS. 1 and 2 as obtained by use of a transmission electronmicroscope, in the present branched vapor-grown carbon fiber of thepresent invention, an individual fiber filament does not have nodules ona branched portion thereof, and a central hollow portion extendsthroughout the filament including the branched portion.

EXAMPLE 5

[0107] In the same manner as in Example 1, vapor grown carbon fiberhaving an average outer diameter of 0.04 μm, an aspect ratio of about40, and a compressed specific resistance of 0.015 Ω·cm was obtained.From observations by use of a transmission electron microscope (TEM), itwas confirmed that the carbon fiber contained 15 mass % of branchedcarbon fiber filaments.

[0108] 0.5 mass part of the carbon fiber was added to a resin solutioncontaining a polyester resin (4.5 mass parts) and methyl ethyl ketone(MEK) (95 mass parts), and the carbon fiber was dispersed in thesolution by use of a paint shaker, to thereby yield an electricallyconductive transparent composition. The composition was applied onto aglass plate by use of a spin-coater so as to attain a film thickness of0.1 μm, and then dried at 150° C. for 1.5 hours. The transmittance at600 nm and surface resistivity of the resultant coating were measured.The surface resistivity and transmittance of the coating were 2,000 Ω/□and 80%, respectively.

EXAMPLE 6

[0109] In a manner similar to that of Example 5, a coating was formed byuse of the same vapor grown carbon fiber as in Example 5; i.e., vaporgrown carbon fiber having an average outer diameter of 0.04 μm, anaspect ratio of about 40, and compressed specific resistance of 0.015Ω·cm (0.25 mass arts) and carbon black (Ketjen Black EC, product ofAKZO) (0.25 mass parts). The surface resistivity and transmittance ofthe coating were 1,500 Ω/□ and 75%, respectively.

EXAMPLE 7

[0110] The same vapor grown carbon fiber as in Example 5; i.e., vaporgrown carbon fiber having an average outer diameter of 0.04 μm and anaspect ratio of about 40 was mixed with 4 mass parts of boron carbide(B₄C), and the resultant mixture was subjected to heat treatment at2,800° C. in an atmosphere of inert gas. After heat treatment wascompleted, the boron content of the carbon fiber was 1.8 mass %, thecompressed specific resistance of the carbon fiber was 0.008 Ω·cm, andthe interlayer distance d₀₀₂ was 0.3375 nm. In a manner similar to thatof Example 5, a coating was formed by use of 0.5 mass part of theresultant vapor grown carbon fiber. The surface resistivity andtransmittance of the coating were 1,500 Ω/□ and 80%, respectively.

EXAMPLE 8

[0111] The vapor grown carbon fiber obtained in the same manner as inExample 1; i.e., vapor grown carbon fiber having an outer diameter of0.08 μm, an aspect ratio of about 40, and a compressed specificresistance of 0.015 Ω·cm was treated at 35° C. in a fluorine (F₂)atmosphere. In a manner similar to that of Example 5, a coating wasformed by use of the resultant vapor grown carbon fiber. The surfaceresistivity and transmittance of the coating were 2,000 Ω/□ and 90%,respectively.

EXAMPLE 9

[0112] Vapor grown carbon fiber having an outer diameter of 0.08 μm, anaspect ratio of about 40, and a compressed specific resistance of 0.018Ω·cm was activated with carbon dioxide gas, to thereby yield activatedcarbon fiber having a specific surface area of 2,000 m²/g. The carbonfiber was immersed in 10% sulfuric acid for one hour, and then washedwith water. Subsequently, the resultant carbon fiber was added to asodium aluminate solution, to thereby form an alumina film (25 mass %)on the surface of the carbon fiber. In a manner similar to that ofExample 5, a coating was formed by use of the resultant vapor growncarbon fiber. The surface resistivity and transmittance of the coatingwere 4,000 Ω/□ and 95%, respectively.

COMPARATIVE EXAMPLE 3

[0113] In a manner similar to that of Example 5, a coating was formed byuse of the vapor grown carbon fiber obtained in the same manner as inExample 1; i.e., vapor grown carbon fiber having an outer diameter of0.5 μm, an aspect ratio of about 40, and a compressed specificresistance of 0.022 Ω·cm. The surface resistivity and transmittance ofthe coating were 2,500 Ω/□ and 35%, respectively.

COMPARATIVE EXAMPLE 4

[0114] In a manner similar to that of Example 5, a coating was formed byuse of carbon black (Ketjen Black EC, product of AKZO) having a BETspecific surface area of 1,270 m²/g. The surface resistivity andtransmittance of the coating were 3,000 Ω/□ and 10%, respectively.

INDUSTRIAL APPLICABILITY

[0115] The vapor grown carbon fiber containing branched carbon fiber ofthe present invention has a very small outer diameter, each fiberfilament having a hollow cylindrical structure in which a central hollowportion extends throughout the filament including a branched portionthereof, which carbon fiber has high electrical conductivity and heatconductivity. Therefore, when the carbon fiber is added to a materialsuch as resin or rubber or to electrodes of various batteries, thecarbon fiber filaments are dispersed so as to form a network structure,to thereby enhance electrical conductivity and heat conductivity of sucha material. In addition, since the present carbon fiber has a diametersmaller than that of conventional carbon fiber, even when the presentcarbon fiber is incorporated into a resin in a relatively large amount,transparency inherent to the resin can be maintained, and a transparentcoating, film, or sheet of high electrical conductivity can be formedfrom the resin.

[0116] The electrically conductive composition of the present inventiondoes not lose transparency inherent to a resin and exhibits excellentelectrical conductivity. In general, an electrically conductivecomposition containing carbon powder or conventional carbon fiber haslow transparency. In contrast, the electrically conductive compositionof the present invention has both high electrical conductivity and hightransparency, since transparency of the resin is barely lowered evenwhen the amount of carbon fiber incorporated.

1. Branched vapor-grown carbon fiber having an outer diameter of 0.5 μmor less and an aspect ratio of at least 10, each fiber filament having ahollow cylindrical structure, characterized by having a compressedspecific resistance of 0.02 Ω·cm or less.
 2. Branched vapor-grown carbonfiber as claimed in claim 1, which has an outer diameter of 0.05-0.5 μm,a length of 1-100 μm, and an aspect ratio of 10-2,000.
 3. Branchedvapor-grown carbon fiber as claimed in claim 1, which has an outerdiameter of 0.002-0.05 μm, a length of 0.5-50 μm, and an aspect ratio of10-25,000.
 4. Branched vapor-grown carbon fiber as claimed in claim 2 or3, which has a compressed specific resistance of 0.018 Ω·cm or less,each fiber filament having a structure such that a central hollowportion extends throughout the filament including a branched portionthereof.
 5. Branched vapor-grown carbon fiber as claimed in claim 4,which comprises, in an amount of at least 10 mass %, branched carbonfiber, each fiber filament having a structure such that a central hollowportion extends throughout the filament including a branched portionthereof.
 6. Branched vapor-grown carbon fiber as claimed in claim 1,which further comprises boron.
 7. Branched vapor-grown carbon fiber asclaimed in claim 6, which comprises boron in an amount of 0.01-5 mass %.8. Branched vapor-grown carbon fiber as claimed in any one of claims 1to 7, which has a heat conductivity of at least 100 kcal (mh° C.)⁻¹. 9.Branched vapor-grown carbon fiber as claimed in claim 8, which has aheat conductivity of at least 100 kcal (mh° C.)⁻¹ when the fiber iscompressed so as to attain a bulk density of 0.8 g/cm³.
 10. A processfor producing branched vapor grown carbon fiber as claimed in claim 1,by thermal decomposition of an organic compound with a transition metalcatalyst, characterized by spraying droplets of organic compoundcontaining 5-10 mass % of a transition metal element or its compound ona heating furnace wall to allow reaction to form carbon fiber filamentson the furnace wall, burning the recovered filaments at 800-1,500° C. ina non-oxidative atmosphere, and heating them at 2,000-3,00° C. toperform graphitization treatment in a non-oxidative atmosphere.
 11. Theprocess as claimed in claim 10, wherein the heating for graphitizationtreatment is performed after doping with boron or at least one boroncompound selected from the group consisting of boron oxide, boroncarbide, boric ester, boric acid or its salt, and organic boroncompounds as a crystallization promotion compound in an amount of 0.1-5mass % in terms of boron.
 12. An electrically conductive transparentcomposition comprising a resin binder and carbon fiber incorporated intothe binder, characterized by having transparency and comprising vaporgrown carbon fiber having an outer diameter of 0.01-0.1 μm, an aspectratio of 10-15,000, and a compressed specific resistance of 0.02 Ω·cm orless.
 13. The electrically conductive transparent composition as claimedin claim 12, wherein the carbon fiber is vapor grown carbon fiber havingan outer diameter of 0.05-0.1 μm or less, a length of 1-100 μm, and anaspect ratio of 10-2,000, each fiber filament having a hollowcylindrical structure.
 14. The electrically conductive transparentcomposition as claimed in claim 12, wherein the blending amount of vaporgrown carbon fiber is 5-40 mass % of the total composition.
 15. Theelectrically conductive transparent composition as claimed in claim 12,which has a surface resistivity of 10,000 Ω/□ or less.
 16. Theelectrically conductive transparent composition as claimed in claim 12,which has a surface resistivity of 5-10,000 Ω/□, and a transmittance ofat least 60% when the composition is formed to have a thickness of 0.5μm.
 17. The electrically conductive transparent composition as claimedin claim 12 or 13, wherein the carbon fiber is vapor grown carbon fiberhaving an interlayer distance (d₀₀₂) of carbon crystal layers of 0.339nm or less and a compressed specific resistance of 0.018 Ω·cm or less.18. The electrically conductive transparent composition as claimed inclaim 13, wherein the branched vapor grown carbon fiber has a compressedspecific resistance of 0.018 Ω·cm or less, each fiber filament thereofhaving a structure such that a central hollow portion extends throughoutthe filament including a branched portion thereof.
 19. The electricallyconductive transparent composition as claimed in claim 18, wherein thecarbon fiber comprises, in an amount of at least 10 mass %, branchedvapor-grown carbon fiber, each fiber filament having a structure inwhich a central hollow portion extends throughout the filament includinga branched portion thereof.
 20. The electrically conductive transparentcomposition as claimed in claim 12 or 13, wherein the vapor grown carbonfiber comprises boron or a combination of boron and nitrogen in anamount of 0.01-3 mass %.
 21. The electrically conductive transparentcomposition as claimed in claim 12 or 13, wherein the vapor grown carbonfiber comprises fluorine in an amount of 0.001-0.05 mass %.
 22. Theelectrically conductive transparent composition as claimed in claim 12or 13, wherein the vapor grown carbon fiber is coated with 20-70 mass %aluminum oxide.
 23. The electrically conductive transparent compositionas claimed in claim 12 or 13, which comprises carbon black together withthe vapor grown carbon fiber.
 24. An electrically conductive transparentmaterial formed from an electrically conductive transparent compositionaccording to claim 12 or
 13. 25. The electrically conductive transparentmaterial as claimed in claim 24, which assumes a form of coating, filmproduced through spraying, film, or sheet.